bpf: Pack struct bpf_fib_lookup

The struct bpf_fib_lookup is supposed to be of size 64. A recent commit
59b418c7063d ("bpf: Add a check for struct bpf_fib_lookup size") added
a static assertion to check this property so that future changes to the
structure will not accidentally break this assumption.

As it immediately turned out, on some 32-bit arm systems, when AEABI=n,
the total size of the structure was equal to 68, see [1]. This happened
because the bpf_fib_lookup structure contains a union of two 16-bit
fields:

    union {
            __u16 tot_len;
            __u16 mtu_result;
    };

which was supposed to compile to a 16-bit-aligned 16-bit field. On the
aforementioned setups it was instead both aligned and padded to 32-bits.

Declare this inner union as __attribute__((packed, aligned(2))) such
that it always is of size 2 and is aligned to 16 bits.

  [1] https://lore.kernel.org/all/CA+G9fYtsoP51f-oP_Sp5MOq-Ffv8La2RztNpwvE6+R1VtFiLrw@mail.gmail.com/#t

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: e1850ea9bd9e ("bpf: bpf_fib_lookup return MTU value as output when looked up")
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240403123303.1452184-1-aspsk@isovalent.com

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 79c548276b6b331db4b41782ae1b7dd74e7809e3..6fe9f11c8abe0074b2b124564e2283b2b41f7460 100644 (file)
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
 
                /* output: MTU value */
                __u16   mtu_result;
-       };
+       } __attribute__((packed, aligned(2)));
        /* input: L3 device index for lookup
         * output: device index from FIB lookup
         */

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 79c548276b6b331db4b41782ae1b7dd74e7809e3..6fe9f11c8abe0074b2b124564e2283b2b41f7460 100644 (file)
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
 
                /* output: MTU value */
                __u16   mtu_result;
-       };
+       } __attribute__((packed, aligned(2)));
        /* input: L3 device index for lookup
         * output: device index from FIB lookup
         */